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Physical Review A

Stimulated laser Raman processes in low-density Rydberg matter: Wave number and intensity blueshifts

Leif Holmlid

Published Dec 14, 2000• 33 citations
Publisher

Abstract

Wave-number blueshifts of single-mode laser light in low-density Rydberg matter are found to be due to stimulated Raman processes. The Rydberg matter is mainly formed from Rydberg states of K atoms. The lasers used are mW continuous-wave lasers in the ir, lead-salt tunable diode lasers. Of special significance is the blueshifting observed as shifts of Fabry-Perot etalon fringes of the order of 0.05 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. This blueshifting is explained as due to complete conversion to anti-Stokes radiation through higher-order stimulated Raman shifting by extremely small quanta from the near continuum of energy levels in the metallic Rydberg matter phase. Complicated strong shifts of intensity to larger wave numbers in the laser modes are also observed, due to coupling of laser side modes to the anti-Stokes waves. The high efficiency of the stimulated Raman process is partly due to the weak coupling to other degrees of freedom.

DOI
10.1103/physreva.63.013817

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Annual Meeting Optical Society of America•Jan 1, 1989•0 citations
Wave-number blueshifts of single-mode laser light in low-density Rydberg matter are found to be due to stimulated Raman processes. The Rydberg matter is mainly formed from Rydberg states of K atoms. The lasers used are mW continuous-wave lasers in the ir, lead-salt tunable diode lasers. Of special significance is the blueshifting observed as shifts of Fabry-Perot etalon fringes of the order of 0.05 cm\ensuremath−1{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}cm\ensuremath−1. This blueshifting is explained as due to complete conversion to anti-Stokes radiation through higher-order stimulated Raman shifting by extremely small quanta from the near continuum of energy levels in the metallic Rydberg matter phase. Complicated strong shifts of intensity to larger wave numbers in the laser modes are also observed, due to coupling of laser side modes to the anti-Stokes waves. The high efficiency of the stimulated Raman process is partly due to the weak coupling to other degrees of freedom.
We report the first Raman study of the Rydberg matter form of Cs, using a ns pulsed dye laser. Electronic Raman anti-Stokes and mixed Stokes processes are detected in the range 290--920 nm after nonresonant excitation at 564 nm. No fluorescence is observed. The anti-Stokes transitions are from a high circular Rydberg state to a low circular state 8J8J8J in Cs+{\mathrm{Cs}}^{+}Cs+, close to 4F4F4F in Cs. The Stokes transitions are mixed in one more step with the laser radiation, and go from low circular states to Rydberg matter states. The transitions observed agree well with transitions in Cs+{\mathrm{Cs}}^{+}Cs+.
When an almost single-mode IR laser beam is reflected from an IR window with a deposit of Rydberg Matter on the inner, vacuum face, the laser modes appear red-shifted; thus their maxima are observed at lower laser current. This effect is complementary to the previously reported blue-shifts of IR laser modes in transmission through Rydberg Matter. Both types of shifts are due to interaction between the laser modes, now proved by using only two isolated modes both in reflection and transmission. The general process is stimulated Raman scattering. Rydberg Matter is here formed from Rydberg states of K atoms and nitrogen molecules inside a vacuum chamber. A theoretical description is given that involves slow decay or increase in the energy of the photons interacting with the Rydberg Matter, depending on the temperature of this matter. This means that the two isolated modes studied may become almost resonant within certain frequency ranges. The energy increase (frequency blue-shifting) of photons passing hot Rydberg Matter has been reported previously. The description used explains both the red-shifted reflection and the blue-shifted transmission results.
A simple description of the stimulated Raman emission of Stokes radiation, and of the induced absorption found by Stoicheff at the anti-Stokes frequency, is based on the semiclassical theory of radiation. This theory gives the well-known formulas for absorption and stimulated emission when carried to the first order in the time-dependent perturbation due to the interaction of the light beam with the molecules. If there are two light beams of frequencies ωL and ω, the second-order perturbation may be associated with stimulated emission at the Stokes frequency ω=ω−1=ωL—ωR and with induced absorption at the anti-Stokes frequency ω=ω1=ωL+ωR, where ωL is the laser frequency and ωR a Raman-active vibrational frequency; in both cases, there is a molecular transition to the excited vibrational state. The highly directional and very sharp anti-Stokes emission at ω1 arises from the conversion by molecules in intense beams at ωL and ω−1 of two ωL photons into an ω−1, ω1 pair. Like spontaneous emission from excited states, the production of this anti-Stokes radiation requires the full quantum theory of radiation for an adequate description. The emission of ω1 and the observed directional absorption of ω−1 due to the reverse conversion ω−1+ω1→2ωL are explained. The anti-Stokes emission is proportional to the Stokes intensity and to the square of that of the laser, and to the square of the molecular number density; its sharpness is due to the absence of a molecular transition. The generation of photon pairs ω−n+ωn, where ωn=ωL+nωR, can occur for all values of n, but it is most efficient for n=1, due to resonance in the appropriate molecular hyperpolarizability and to the intensity of the Stokes beam at ω−1.
The interaction between Rydberg systems [Rydberg atoms (RA) gas or Rydberg matter (RM)] and coherent light in various frequency bands is considered. It is shown why Rydberg systems possess a greater polarizabilities and, accordingly, linear and nonlinear susceptibilities and strong sensitive to high-1 (circular) Rydberg states in RA and RM. Quantum theory of physical and chemical properties of RA and RM are related with recent obtained results on the optical spectroscopy. The experimental data of Raman processes, wave mixing effects in gas of RA, and their clusters of RM are analyzed. Problem of blueshifts and stimulated electronic laser Raman processes, as well as four wave mixing in RM are discussed.
We have observed fluctuations in the direction of propagation of light pulses generated by stimulated Raman scattering (SRS). This fluctuation is analogous to the pointing fluctuation of laser beams,1 and is due to quantum mechanical uncertainties in the occupation numbers of different spatial modes. When SRS is generated in an interaction region with Fresnel number less than unity, the lowest order spatial mode dominates the generation process, but higher order spatial modes are also weakly and randomly excited. This results in small fluctuations in the wavefront of the light field. The direction of propagation is characterized by the transverse component of an average wave-vector determined by the gradient of the wave-front. We report experimental results by measuring the "center of mass" of the far-field intensity pattern of a single Stokes beam, and the fringe separation of an interference pattern formed by superposing two independently generated Stokes pulses. The experimental results are compared to theory.2
The generation of 145.6-nm radiation by stimulated anti-Stokes Raman scattering in inverted atomic selenium was first reported by Ludewigt et al. 1 We have performed experimental studies of this process in similar excitation conditions, which allow us to compare some of the results in the two studies. The inverted population of the selenium (4p4)1S0 metastable state relative to the (4p4)3P0 state was prepared by photodissociation of COSe vapor using broadband 193-nm radiation from an ArF excimer laser. The pump beam at 205.1 nm was generated by the first-order Stokes shift in deuterium of narrowband (0.27-cm–1), 193-nm radiation from a separate, tunable ArF laser. The pump beam was tuned near the selenium (4p4)1S0–(4p35s)3P1 intermediate resonance to generate tunable, coherent radiation at 145.6 nm. We have studied the process in a variety of experimental conditions. Factors affecting the conversion efficiency and tunability include the initial COSe pressure, the intensities and relative timing of the photolyzing and pump beams, and the presence of buffer gases. The experimental results are compared with theoretical calculations based on a recently developed three-level model2 which incorporates Doppler broadening and arbitrary decay times.